It's worth pointing out that the door to the cage in the photo you give as an example appears to be open.
– AJ Henderson♦Mar 1 '13 at 14:35

@AJHenderson, I can't find a picture that is the one that I want to illustrate but the objective is that when I take a photo of the bird in a cage far away from me and if I use a tele lens, it should be able to take a photo "more" of the bird and "less" of the cage.
– JackMar 4 '13 at 1:53

@Jack - yeah, sorry, I understood what you meant. I just wanted to make sure you weren't looking for results exactly like that image, but more like the samples submitted in some of the answers.
– AJ Henderson♦Mar 4 '13 at 14:04

5 Answers
5

Your example is made up of images from two different situations. It is extremely likely that no lens / camera on earth could achieve the sort of image shown in that situation without "cheating" by using some sort of processing - and probably multiple images - to deal with bar removal.

You CAN achieve extremely good results when you have control over where the camera is located relative to the bars. An object that is closer to the focal point than the focal length of the lens will be defocused and light from it's image that reaches the sensor will be dispersed over some or all of the rest of the image. This causes loss of contrast of the overall image but spreads or removes the image of the bars. The closer the bars are to the sensor plane and the further they are behind a plane one focal length in front of the focal point, the more they are dispersed.

So:

Camera lens front surface almost touching the bars if possible.

Long focal length

Large aperture to decrease depth of field.

Subject as close as reasonably possible.

Lens as close to the bars as is reasonably possible is perhaps the biggest "secret".

Through the bars: This was taken through cage bars the same as are seen in the cage above and at rear. The lens front is almost against the bars. Aperture f/1.8. 50 mm (APSC)

This Toucan was taken at f/6.3, 18mm (APSC)
There is very solid close spaced mesh between it and me.

I you're going to be standing on the ground with cages on poles like in the image you posted then you're out of luck.

In order to throw the cage bars out of focus to the extent to which they don't show up in the image your lens will have to be very close to the cage (touching if possible). You'll need a wide aperture lens with a fairly close focusing distance.

Depending on the shooting distance, the thickness of the wire the cage is made of, and the maximum available aperture on your lens you can try the following:

position the bird as close to the far side of the cage as is possible.

Select the widest aperture (lowest f/number) available.

Move as close to the cage as you can and still keep the bird in focus. It will help if you focus your lens at the shortest focus distance it is capable of and then move toward the bird until it is in focus.

If you are able to use a wide enough aperture and keep the bird in focus while the front of your lens is very close to the cage, the blur from the bars might become diffused enough to be minimally visible.

But the best way is to shoot the bird with nothing between your lens and it.

If the image above is your real-life situation, this will be very very difficult. Why? Chances are that bird in the cage will be so far away that the depth of field on the distance will not be able to blur out the cage bars sufficiently to make them disappear 100%. Your only hope is that the wire on the cages is very thin and that you get a better angle to shoot (=closer up) as well as the bird sitting ideally on the other end of the cage.

Why? The bird has to be possibly far away from the cage to have the DOF make it disappear. The cage is far away, your zoom lens has limited aperture and the angle to the cage forces you to move far away. All 3 factors lower your chances.

Here is an example with a couple of generous assumptions:

Let's assume the cage is 20 cm in diameter. Also, the cages are about at 6 meters height. Let's also assume that you will have ideal conditions to use a F/4 aperture. But how far will you have to be away from the cage? Well to even look into the cage and see the bird on the other side, you will have to see it from almost straight ahead. Let's be again positive and assume a 27 degree angle from the bottom would work. That means you have to have twice the distance on the ground than the cage is high = 12 meters. Assuming the bird is not blocked by the bottom of the cage. The DOF then would be still 24cm - 4 cm of DOF, which is not enough to make the bars disappear completely, since the bird will most likely sit in the center of the cage. With a 24 cm depth of field, your whole cage will be in focus - all bars included. If you need to move further away, the problem becomes worse. Same if there is not enough light or your shutter speed (300mm needs a certain speed to not blur from shake, and the bird might be moving) needs to be too high to get the bird sharp.

You don't want a 500mm or anywhere near. The longer your lens the greater the working distance, this means the bird and the bars will be similar distance from the lens. Closest focus distance of the Canon 500 f/4L is 4.5m assuming the bird is 30cm from the bars, then the distances to bird and bars are 4.5m and 4.2m respectively. The closest focus distance of the Canon 50mm f/1.4 is 45 cm. The distances to the bird and bars can now be 45cm and 15cm, i.e. at 500mm the distance was about the same, but at 50mm the bars are now three times closer than the bird, and hence more out of focus.
– Matt GrumMar 1 '13 at 9:47

@MattGrum true, but if the cage is on a pole already 6m away, your minium focus of 4.5 meters does not really apply.
– uncoveryMar 1 '13 at 10:12

If the cage is on a pole 6m away then you wont be able to throw the near bars sufficiently out of focus no matter what lens you use!
– Matt GrumMar 1 '13 at 10:31